I know this is an older thread, but I just wanted to mention that this particular problem is a very common one newbies face when trying pixel-based animation that also rarely gets addressed.
First off, to "fix" this problem, one starts with solid colored masses (not fully-colored heads, or portions of the body). If the silhouettes of the movement look correct, then you can start adding things like outlines and basic form.
Unfortunately, when trying to add outlines to small resolutions like this, animation can get very "floppy" and "muddy" as MysteryMeat pointed out about the wet or "sand-filled" floppy socks.
Most importantly -- do NOT animate pixel art in Photoshop! Get something like Graphics Gale (free!) or Asesprite, which has OnionSkinning capabilities so that you can see your silhouette (for the moment, I suggest Graphics Gale for animation, but Asesprite is definitely catching up!) Graphics Gale has a keyboard shortcut where you can map the F10, F11, F12 keys to the mouse wheel so you can flip quickly between animation frames. This is how you can check your silhouette more easily.
Finally, regarding "volume" -- please remember that outlines are a "post-processing" effect. They are generated AFTER the volume itself. So when animating the silhouette, your are animating only VOLUME and not outlines or details or lighting or shadow. Only volume. Therefore, no matter what you do, consider how heavy these volumes are as you flip-flop between frames to study them as they move. Ensure they remain of consistent size and proportion to what kind of materials they are.
Sorry for the necro-post -- but this section is not very lively these days, so I wanted to help infuse it with a little know-how if I could. I want to see a little more life here in this section of the forums.